As part of the deal, Tumblr CEO David Karp — who got a windfall
of cash from the deal — will stay at Yahoo for four years at
least and retain much control over the service, much in the same
way Instagram CEO Kevin Systrom does at Facebook. But, as there,
Yahoo will undergird Tumblr’s nascent advertising business with
its large and established infrastructure.

If they treat Tumblr the way Facebook has (so far) treated Instagram, I think this will work out well.